Television receiving system



Mam]! 1940- A. BOUWERS 2,193,857

' TELEVISION RECEIVING SYSTEM Filed Oct. 9, 1937 10cm 1%! CARR/ER osc/zuro/z AMPLIFIER 05750701? M00l/M7Z7R I 4 5 8 l2 /6 HORIZONTAL (l/NE) 20 FREOIIENCYGENERAMR 8 22 10 2 FILTER FILTER /4 VERTICAL (FIELD) msaumcrasusmrok INVENTOR- ALBERT OUWERS fi M A T T ORNEY.

Patented Mar. 19, 1940 UNITED STATES PATENT orricE I 2,193,857 TELEVISION RECEIVING SYSTEM I Albert Bouwers, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application October 9, 1937, Serial No. 168,122

In the Netherlands November 9, 1936 i 4 Claims.

' perpendicular to the scanning lines.

' With the existing television systems it is generally customary to utilize in the transmitter a device by which the picture to be transmitted, or an image of thepicture, is scanned at a uniform speed and wherein a current is obtained which fluctuates in dependence on the brilliancy of the picture elements which are successively scanned while the picture currents thus obtained are modulated on a carrier wave and are trans- 1 tube while the cathode ray beam is moved by at least one device for deflecting the cathode ray beam at a uniform speed over a screen arranged within the tube.

A second method of transmitting pictures consists in scanning the picture to be transmitted at a speed which is determined by the brilliancy of the picture elements of the picture to be transmitted which are successively scanned whereupon in dependence on said speed a carrier wave is modulated and emitted. In the corresponding receiver the oscillations obtained after detection are supplied to the device for deflecting a cathode ray beam in a direction parallel to the scanning sponding picture elements of the picture to be lines with the result that the cathode ray beam is moved at the receiver at. a variable speed over a screen arranged within the tube, said speed being dependent on the brilliancy of the corretransmitted. The use of the last-mentioned sys tem entails a considerable disadvantage which resides in that the time during which an entire line or an entire picture is scanned is not constant so that particular measures are necessary to ensure synchroni'sm between the picture transmitter and receiver. This invention is based on the recognition tha with the use of the last mentioned scanning method, which will hereinafter be referred to as velocity scanning, a material intensification of the contrasts anda considerable increase of I the average quantity of light emitted .from the (c1.17s v.5)

picturereproduced is obtained if use is made of a cathode ray tube having a screen which becomes luminous upon electron bombardment and which has the property that there exists a nonlinear relation between the time during which the electrons strike the screen (hereinafter referred to as time of exposure) and the quantity of light emitted.

The invention is more particularly concerned with a television receiving arrangement in which the cathode-ray tube comprises a screen which becomes luminous by electron bombardment and has the property that a non-linear relation exists between the time during which the electrons strike the screen and the quantity of light emitted.

Cathode ray tubes which have a screen that emits light upon electron bombardment and wherein anon-linear relation exists between the time of exposure and the quantity of light emitted have previously been suggested in themselves for television purposes. One example of a tube wherein the screen has the characteristic curve desired is a tube which has a screen consisting of refractorymaterial which is raised to incandescence by electron bombardment. Tubes having similar characteristics but with different screens may, of course, be used.

The system according to the invention consists, therefore, in a television receiving system of the kind wherein the received oscillations obtained after detection are supplied to the above-mentioned device for deflecting the cathode ray beam in a direction parallel to the scanning lines and wherein the cathode ray tube comprises a screen which becomes luminous when struck by electrons and which has the property that there exists a non-linear relation between the time during which the electrons strike the screen and the quantity of light emitted.

The invention has for its purpose, therefore, to procure an improvement of the television receiving arrangement above referred to, this improvement consisting in that a carrier wave, modulatedby the received picture currents, is supplied to the deflecting means of the device for deflecting the beam of cathode rays in a direction parallel with the scanning lines.

- The invention will be more clearly understood by reference to the accompanying drawing wherein:

Figures 1 and 2 each shows one form of construction of the circuit arrangement according to the invention; and

Figure 3 shows, for example, a response curve for a cathode ray tube screen.

The television receiving arrangement represented in Figure 1 comprises a cathode ray tube 2 equipped with an electrode system for generating a beam of cathode rays and consisting at least of a cathode 4 and an anode 5. To deflecting plates 8 is supplied an alternating voltage which is produced by a device Iii for the deflection of the beam of cathode rays in a direction parallel with the scanning lines, hereinafter called horizontal scanning device, said voltage having a saw tooth curve. In addition two deflecting plates i2 are provided to which is fed a saw tooth potential produced by a device M for deflecting the beam of cathode rays in a direction normal to the scanning lines, this device being hereinafter called vertical scanning device.

The receiving arrangement furthermore comprises an amplifier it to which are supplied the received modulated oscillations and which may be designed for frequency transformationof the received carrier waves. The oscillations occurring in the output circuit of the amplifier [6 are supplied to a detector IS, in the output circuit of which low frequency picture currents and synchronizing impulses occur, which impulses are transmitted for the purpose of synchronizingthe saw tooth voltages produced by the devices l and M. A filter 28 serves to separate the synchronizing impulses and the picture currents, Whereas a filter 22 is provided for splitting up the synchronizing pulses into line pulses and picture pulses. The line impulses which are emitted by the transmitter at the end of each scanning line, are supplied to the horizontal scanning device l0, Whereas the picture'impulses-which occur at the end of the'scanning of each picture, are applied to the vertical scanning device [4. The low frequency picture currents set up in the output-circuit of the detector l8, if desired after having been separated from the synchronizing impulses, are modulated on a carrier wave generated by a local oscillator 24 and the modulated oscillations thus produced are supplied to the deflecting plates 8. The means for modulating the picture currents on the oscillations generated by the local oscillator 24', is designated by 26 in the'drawing.

Figure 2 shows a form of construction in which the modulated oscillations occurring in the output circuit of the amplifier l6 are supplied torthe deflecting plates of a cathode ray tube without interposition of a detector. Otherwise the circuit arrangement exactly corresponds to that represented in Figure l.

The operationof the circuit arrangement set out above is based on the fact that'the cathode ray tube 2 comprises a screen which has the property that a non-linear relation existsv between the time during which the electrons-strike the screen and thequantity of light emitted as a function of the time during which the electrons strike the screen may consequently have a shape as illustrated in Figure 3, where L is the quantity of light emitted and T the time during which the electrons strike the screen, hereinafter called time of exposure.

To the deflecting plates 8 is supplied a saw tooth potential which is generated by the device lll and. serves to deflect the beam of cathode rays in a direction parallel with the scanning lines, the period of which determines the scanning time of one scanning line. When superposing on this voltage a non-modulated oscillation, which is generated by the oscillator 24 whose frequency is preferably a multiple of the saw tooth potential generated by the device l0, the movement of the scanning beam in scanning each scanning line will be accelerated and retarded during each half cycle of the impressed oscillation. However, the total scanning time of a scanning line remains constant and equal to the period of the saw tooth potential generated by the device I 0. 10

The time during which the electrons strike the screen is inversely proportional to the speed at which the scanning beam moves over the screen. During that part of the half cycle of the impressed oscillation, in which the movement of the scanning beam is accelerated, the time during which the electrons strike the screen is short, and during the retarded part of the half cycle of the impressed oscillation the time of exposure is long. If the curve indicating the quantity of light L emitted under bombardment by electrons as a function of the time T has the shape illustrated in Figure then the emitted quantity of light increases with an increase of the time of exposure from the value T1 to T2, i. e., duringthe retarded part of the half-cycle of the impressed oscillation, to a larger extent than the quantity of light emitted during the accelerated part 'of the half-cycle decreases due to the decrease of the time of exposure from T1 to T3n30 From this it results that the average quantity of light emittedindicated by the dash-and-dot line in Figure 3, is appreciably larger than if the scanning beam were moved at a uniform speed over the screen in which case the average quan-l. 5 tity of light emitted would be determined by the time ofexposure T1 corresponding tosaid speed.

With an increase and with a decrease respectively'of the amplitude of the oscillations fed to the deflecting plates 8 of the cathode ray tube 2110 the acceleration and the retardation of the beam of cathode rays during each half cycle of the impressed oscillations increases and decreases respectively in accordance with the increase and .clecrease respectively of the amplitude of thei oscillations; in other words the average quantity of light emitted during each cycle of the oscillations impressed on the deflecting plates 8 varies in accordance with the amplitude of these oscillations since the increase of the quantity of light;.- emitted during that part of the half cycle of the impressed oscillations, in which the scanning beam is retarded, is always appreciably larger than the decrease during that part of the half c 'cle in which the scanning beam is accelerated In the circuit arrangement according to the invention the amplitude of the oscillations supplied to the deflecting plate 8 varies in accordance with the low frequency picture currents'which are modulated on said oscillations so that a variation of the average quantity of light emitted during each cycle of the carrier wave is obtained, which variation depends on the low frequency picture currents modulated on the carrier wave. at

. Since the increase of the average quantity of light emitted during that part of the half cycle inwhich the scanning speed is retarded is'much larger than the decrease of the average quantity of light emitted during that part of the half cycle in which the scanning speed is increased, an appreciable intensification of the contrasts in reproducing the picture in the manner set out above is obtained.

The explanation of the phenomenon that, withu,

the use of a cathode ray tube as above referred to in a television receiver wherein velocity scanning is utilized, an appreciable intensification of the contrasts and an increase of the average quantity of light emitted from the picture reproduced is obtained is based on the following.

The time during which the electrons strike the screen is inversely proportional to the velocity with which the screen is scanned. With a bright picture element the time during which the electrons strike the screen is long and with a dark picture element'the time of exposure is short or vice versa. If the curve which represents the quantity of light L which is radiated upon electron bombardment as a function of the time of exposure has a shape as shown in Figure 3, the quantity of light emitted increases, with an increase of the time of exposure from the value T1 to T2, that is to say during the reproduction of a bright picture element, more strongly than the quantity of light emitted decreases, due to the decrease of the time of exposure from T1 to T3, during the reproduction of a dark picture element. It results therefrom that the contrasts between the bright picture elements and the dark ones are intensified while in addition the average quantity of light emitted, which is indicated in Figure 3 by a dot-and-dash line, is considerably larger than with the use of a screen with which the relation between the time of exposure and the;

quantity of light emitted is linear and whereby consequently the average brilliancy remains constant.

' It is not imperative that the curve indicating the quantity of light emitted as a function of the time of exposure should have a shape asrep're-' sented in Figure 3,'where the quantity of light emitted increases more than proportionally with the time of exposure. If in reproducing a bright picture element the amplitude of the carrier wave supplied to the deflecting plates 8 increases, and

in reproducing a dark picture element the amplitude of the carrier wave fed to the deflecting the quantity of light emitted decreases more than proportionally with time. In either case the contrasts betweenthe light and dark picture elements are also increased, whereas the average quantity of light emitted is larger than when using a television receiving arrangement not in accordance with this invention, in which the lowfrequency picture currents control the intensity of the beam of cathode rays.

It is to be understood that, although electrostatic deflection means are shown, any form of deflection, whether electrostatic, electrodynamic, or a combination of these, may be used.

Various modifications may be made in the invention without departing from the spirit and the scope thereof, and it is desired that any and all such modifications shall be considered within the purview of the invention except as limited by the hereinafter appended claims.

I claim as my invention:

1. A television receiving arrangement comprising a cathode ray tube, means for producing a cathode ray beam in the tube, a screen adapted to emit light when subjected to electronic bombardment and having the property that a nonlinear relationship exists between the time during which the electrons strike the screen and the quantity of light emitted therefrom, means for deflecting the cathode raybeam inone direction at a constant frequency, means for generating a substantially high frequency carrier wave, means for modulating the carrier wave by the received picture signals, and means for very rapidly varying the rate of deflection in accordance with the intensity of the modulated carrier wave.

2. A circuit arrangement as claimed in claim 1, in which the frequency of the carrier wave which is supplied to the deflecting means of the device for deflecting the beam. of cathode rays and modulated by the received picture currents is a multiple of the frequency of the current or voltage supplied to the deflecting means by the said de vice.

3. A circuit arrangement as claimed in claim 1, in which the screen of the cathode ray tube consists of a highly refractory material which is brought to incandescence under bombardment by electrons.

l. A television receiving arrangement comprising a cathode ray tube, means for producing a stream of electrons in said tube, said tube includ ing a screen adapted to emit light when bombarded by the stream of electrons and which has the property that a non-linear relationship exists between the time during which the electrons strike the screen and the quantity of light emitted-thereby, means for repeatedly deflecting the beam of electrons in substantially perpendicular directions at different constant frequencies to scan said screen, means for rapidly accelerating and decelerating the rate of deflection of the beam in one direction at a constant rate, and means for controlling the amount of acceleration or deceleration in accordance with the received picture signals.

ALBERT BOU'WERS. 

